This invention relates to improved polyethylene homopolymers and random copolymers with controlled narrow molecular weight distributions, and a process for producing such compositions. The process is particularly useful in the production of polyethylene homopolymers and random copolymers for use in a wide variety of thermoplastic applications. The present invention also relates to such polyethylene compositions produced by a stable free radical polymerization process.
The polyethylene compositions of the present invention may be formed into a variety of thermoplastic products, for example by known processes such as coating, rotational molding, thermoforming, extruding, injection molding and blow molding processes. Examples of such thermoplastic products include kitchenware, coating films over paper or aluminum foil for packaging, linings for chemical drums and water piping, and uses in electrical wire and cable insulation. In addition, because of its good chemical resistance, polyethylene is found in chemical ware and as a component of electrical apparatus.
The production of polyethylene polymers having varying structure and characteristics is known in the art. For example, low density polyethylene (LDPE) may be made by polymerizing ethylene gas under a pressure of 1,000 to 3,000 bar at temperatures between 120.degree. C. and 350.degree. C. with 0.05 to 0.1% oxygen or peroxide initiators. This reaction may be performed in a tubular reactor of about one mile in length by a continuous process. The peroxide initiator, such as benzoyl peroxide, is added as a solution in food grade hydrocarbon solvent at the beginning of the reaction and also injected into the middle of the reactor. This process produces low density polyethylene polymer with a high degree of random branching, where the length of each branch is only about 4 carbon units due to back biting of the chains. The standard conversion of monomer to polymer is typically in the range of 15 to 25 percent with limited control over the molecular weight of the polymer and no control over the polydispersity. As a consequence the upper range of the molecular weight is limited to around 70,000.
A high density linear polyethylene polymer with a molecular weight of about 50,000 can be produced in a solution free radical polymerization process in xylene at from 150.degree. C. to 180.degree. C. at pressures of greater than 35 bar, using Chromium and aluminum silica catalysts. Furthermore, Ziegler-Natta catalysts can be used to synthesize low-pressure high density polyethylene polymers. In this process, ethylene is introduced into a dispersion of mixed catalysts such as TiCl.sub.4 and aluminum alkyl at a pressure of from 1 to 50 bar and a temperature of from 20.degree. to 250.degree. C. The ethylene gas polymerizes into almost unbranched, linear high density polyethylene of medium to high molecular weight with only a small number of short side-chains.
The use of stable free radicals as inhibitors of free radical polymerization is known, for example as described in G. Moad et al., Polymer Bulletin, vol. 6, p. 589 (1982). Studies have also reported on the use of stable free radicals as inhibitors of free radical polymerization performed at low temperatures and at low monomer to polymer conversation rates. See, for example, G. Moad et al., Macromol Sci.-Chem., A17(1), 51 (1982).
Free radical polymerization processes are also generally known in the art. For example, Roland P. T. Chung and David H. Solomon, "Recent Developments in Free-Radical Polymerization--A Mini Review." Progress in Organic Coatings, vol. 21, pp. 227-254 (1992), presents an overview of the free radical polymerization process, with an emphasis on recent developments.
U.S. Pat. No. 5,322,912 to Georges et. al. discloses a free radical polymerization process for the preparation of thermoplastic resins. The thermoplastic resins are disclosed as having a molecular weight of from 10,000 to 200,000 and a polydispersity of from 1.1 to 2.0. The process comprises heating a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form a thermoplastic resin with a high monomer to polymer conversion ratio, and then cooling said mixture. The polymerization process is carried out at a temperature of from 60.degree. to 160.degree. C. and at a relatively low pressure of about 60 psi (about 4 bars). The process optionally comprises isolating the thermoplastic resin or resins and washing and drying the thermoplastic resin. The patent also discloses the preparation of random and block copolymer thermoplastic resins using the free radical polymerization process. Resins produced by the disclosed process are described as having a narrow molecular weight distribution, and a modality that is controlled by the optional sequential addition of the free radical initiator and stable free radical agent. The patent does not disclose the production of polyethylene homopolymers and random copolymers.
U.S. Pat. No. 5,100,978 to Hasenbein et al. discloses a free radical polymerization process for producing polyethylene homopolymers and copolymers. The polyethylene copolymers include predominant amounts of ethylene and minor amounts of comonomers that are polymerizable with ethylene. The free radical polymerization process is conducted at a pressure of from 1,500 to 5,000 bar and at a temperature of from 40.degree. C. to 250.degree. C. The process includes at least three separate polymerization stages with fresh initiator being introduced in each stage, wherein polymerization proceeds in the presence of the initiator. The process results in a polyethylene homopolymer or copolymer having a density of more than 925 kg/m.sup.3.
U.S. Pat. No. 4,581,429 to Solomon et. al. discloses a free radical polymerization process that controls the growth of polymer chains to produce short chain or oligomeric homopolymers and copolymers, including block and graft copolymers. The process employs an initiator having the formula, in part, equal to .dbd.N--O--X where X is a free radical species capable of polymerizing unsaturated monomers. The molecular weights of the polymer products obtained are generally from about 2,500 to 7,000 and have polydispersities generally of from about 1.4 to 1.8. The reactions typically have low monomer to polymer conversion rates and use relatively low reaction temperatures, of less than about 100.degree. C., and use multiple stages.
U.S. Pat. No. 4,777,230 to Kamath discloses a free radical polymerization process for producing polymers, wherein monomers are dissolved in solvent with, polymerization initiators (such as peroxide initiators) and an optional chain transfer agent. The polymerization process is conducted at a temperature of from about 90.degree. C. to about 200.degree. C. The resultant polymers have a molecular weight distribution of from about 1.5 to about 2.5, and an average molecular weight of less than about 4,000.
Neither of the latter two patents discloses the production of polyethylene homopolymers and random copolymers at high temperature and pressure using a stable free radical polymerization process.
A problem with conventional polyethylene polymerization processes, however, is that they do not allow for the narrow control of the molecular weight distribution of the polymer or copolymer by a free radical process. The advantage of narrowing the molecular weight distribution of polyethylene polymers is reflected in the performance properties of the resultant polymer. A focus in the thermoplastic resin industry has thus been to develop new grades of polyethylene polymers by reducing the molecular weight distribution of the polymer.
Among the advantages of a narrower molecular weight distribution is a decrease in the temperature at which the polymer may be later processed. Because longer chain polymers require more energy to soften and mold the polymers, the processing temperature may be decreased as the molecular weight distribution is narrowed. This factor is especially advantageous when working with resins of high shear viscosity. A decrease in the low molecular weight fraction of the polymer observed as a low molecular weight tail in the molecular weight distribution also results in less volatiles when molding the polymer, thus producing environmentally friendly and safer materials. In addition, eliminating low molecular weight fractions will also contribute to strengthening of the material. Furthermore, when using polyethylene polymers having a narrower molecular weight distribution, it is possible to produce thinner films of the polymer, without detriment to the film's characteristics and properties.
It has been demonstrated that stable free radical polymerization processes can provide precise control over the molecular weight distribution of polymer chains. For example, U.S. Pat. No. 5,322,912, described above, describes a polymerization process that uses stable free radicals to provide thermoplastic resins having a narrow molecular weight distribution. Although it is not desired to be limited by theory, it is believed that when polymerization reaction processes are performed at temperatures above about 100.degree. C., all of the polymer chains are initiated at about the same time. Reversible coupling of the polymer chains by the stable free radical dramatically reduces termination by irreversible coupling. Therefore, control of the reaction enables the formation of polymer chains having a precise molecular weight and a narrow molecular weight distribution.